As noted above, in 1986, the U.S. tuna fleet reached the 

 allowable mortality level of 20,500 and was required to cease 

 setting for tuna on schools of porpoise. There are several 

 possible reasons why the quota was reached. One is that the 

 tuna fleet experienced an increased number of problem sets 

 which resulted in abnormally high levels of deaths. Another 

 possible explanation was the record high tuna catch rate of 

 approximately 25 tons of tuna per set. Large schools of tuna 

 may be associated with large schools of porpoise and, therefore, 

 more porpoise than usual may have been encircled per set. 

 The large number of tuna in the net may also have contributed 

 to the high mortality by making it more difficult to release 

 porpoise during the backdown procedure. 



Porpoise mortality in the U.S. tuna fishery during 1987 

 was significantly less than in 1986 with a total estimate of 

 13,992 porpoise killed. In 1987, the catch rate of tuna was 

 somewhat below 20 tons of tuna per set, well below the record 

 level of 1986, suggesting that a lower incidental mortality 

 of porpoise may be correlated with a lower tuna catch per unit 

 effort. Porpoise mortality in the U.S. tuna fleet for 1988 

 again approached the maximum allowable level, with an estimated 

 total mortality of 19,712. As in 1986, the fleet experienced 

 a relatively high percentage of problem sets, those that 

 account for the highest porpoise mortality. The catch rate 

 of tuna in 1988 was similar to that for 1987, with an average 

 of approximately 19 tons of tuna caught per set in each year. 



The 1984 amendments to the Marine Mammal Protection Act 

 established a quota of 2,750 for the eastern spinner stock. 

 In 1987, despite a relatively low overall mortality of marine 

 mammals, it was estimated that 2,688 eastern spinner dolphins 

 had been killed. In comparison, during 1986, when the overall 

 quota of 20,500 porpoise was reached, the estimated mortality 

 of eastern spinners was only 1,608. The incidental take of 

 eastern spinner dolphins was again high in 1988. The quota 

 for this stock was exceeded for the first time with an estimated 

 mortality of 2,832 by the U.S. fleet. One possible explanation 

 for the high eastern spinner mortality during the past two 

 years is that tuna, and, hence, fishing effort, were 

 concentrated in the area of the eastern tropical Pacific 

 Ocean where eastern spinner dolphins are more abundant. 



While the number of sets on porpoise per year by the 

 U.S. fleet has fluctuated over the past three years, there 

 has been a declining trend in the percentage of tuna caught 

 in such sets. During 1986, 94 percent of the tuna caught by 

 U.S. fishermen was taken in sets on porpoise. In 1987, sets 

 on porpoise accounted for 78 percent of the U.S. tuna catch 

 and, in 1988, only 64 percent of the tuna caught by the U.S. 

 fleet was from marine mammal sets. 



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